1. Field of the Invention
The present invention relates to a piezoelectric film device that uses inversion of a polarization vector of a piezoelectric film and is applied to a positioning device or the like, and relates to a driving method of the piezoelectric film device
2. Background Art
Recently, a piezoelectric element has been used as a driving element of a positioning device and applied to various actuators. That is because the piezoelectric element has high conversion efficiency from electric energy to mechanical energy, can be driven by low power consumption, generates a small amount of heat, and does not cause magnetic interference.
However, the piezoelectric element has a problem that the relation between applied voltage and a displacement amount of the piezoelectric element has hysteresis and hence the displacement amount does not uniquely correspond to the applied voltage.
A driving method of minimizing the hysteresis of the piezoelectric element is disclosed by Japanese Patent Unexamined Publication No. H3-256375 (patent document). FIG. 19 is a characteristic diagram showing the relation between the displacement amount of a conventional piezoelectric element and applied voltage.
In FIG. 19, minimum voltage applied to the piezoelectric element is denoted as Vmin, and maximum voltage is denoted as Vmax. When the applied voltage is varied as Vmin→V1→Vmax→V1→Vmin, the displacement draws the hysteresis of A→C→B→D→A and hence the displacement is not uniquely determined. When the applied voltage is monotonically increased from Vmin as Vmin→V1→Vmax or monotonically decreased from Vmax as Vmax→V1→Vmin, however, respective voltage-displacement characteristic curves are A→C→B and B→D→A and the displacement corresponds to the applied voltage one-to-one. When a previously applied voltage (hereinafter referred to as previous voltage) before application of voltage V1 to the piezoelectric element is assumed to be Vmin, the displacement is XC. When the previous voltage is assumed to be Vmax, the displacement is XD. While, for obtaining displacement XC, the applied voltage is set at V1 for previous voltage Vmin, or set at V4 for previous voltage Vmax. When the previous voltage is always set at Vmin or Vmax, the displacement is uniquely determined by applied voltage.
By the technique described above, the piezoelectric element can be uniquely positioned even when the piezoelectric element has hysteresis.
In the conventional driving method of minimizing hysteresis of the piezoelectric element, however, previous voltage Vmin or Vmax must be always applied before causing a desired displacement. In this case, displacement corresponding to the previous voltage occurs, so that desired displacements cannot be continuously obtained. When two positions of the desired displacement and the displacement determined by the previous voltage are controlled, for example, for retaining at least one displacement, voltage corresponding to the displacement must be continuously applied.
In the conventional driving method, any example of driving a piezoelectric element by inverting the polarization direction thereof is not included. An actuator using a conventional piezoelectric element is driven in a state where the polarization direction is kept the same. On the contrary, in a ferroelectric memory or the like, it is used in a state where the polarization direction is consciously inverted. However, the inversion of the polarization direction is limited to elements using not piezoelectric property but ferroelectric property.
The relation between the displacement in the film direction and the applied voltage at the tip of a unimorph-type cantilever formed of gold (Au) electrode/lead zirconate titanate (PZT) film/platinum (Pt) electrode/magnesium oxide (MgO) single-crystal substrate is disclosed in “Measurement of transverse piezoelectric properties of PZT thin films”, “SENSORS AND ACTUATORS A: PHYSICAL”, vol. 107 Issue 1, 1 Oct. 2003, p 68-p 74 (non-patent document). The PZT film shown in this document is estimated to be a complete c-axis orientation film. This document also shows that there is a high linearity between the applied voltage and the displacement. When alternating voltage of ±35V and 10 Hz is applied to the unimorph-type cantilever, the displacement behavior of the tip thereof forms a butterfly shaped loop, and the loop is asymmetric with respect to the polarization of the voltage.
This document shows that, when alternating voltage with simple sine wave is continuously applied to the conventional unimorph-type cantilever, the displacement asymmetric with respect to the polarization of the applied voltage occurs. However, the document does not show a displacement state of the cantilever obtained when the application of the alternating voltage is stopped. Regarding the voltage application, simple alternating voltage is continuously applied, but an applying method of voltage suitable for complex positional control is not described. Only the configuration of a unimorph-type cantilever having a film and a substrate used for forming the film is disclosed. A configuration where the substrate used in forming the film is removed or a configuration deformed by principles other than the unimorph-type is not disclosed.